1. Field of the Invention
The present invention relates to a parking brake apparatus for applying a brake force to wheels of a parked vehicle. More particularly, the present invention is related to an electrically operated parking brake apparatus for applying the brake force to the wheels through the driving force of an electric motor without relying upon the force applied by the hand or foot of a driver.
2. Description of the Related Art
JP-A-2000-309255 discloses an electrically operated parking brake apparatus with a screw mechanism having in turn a screw-threaded shaft adapted to be rotationally driven by an electric motor, and a nut member adapted to mesh with the screw-threaded shaft. When the screw-threaded shaft is rotated, the nut member is moved along the screw-threaded shaft, whereby a brake operating force is transmitted to wheel brakes via cables connected to the nut member.
Since the electrically operated parking brake apparatus disclosed in JP-A-2000-309255 is of a type in which cables connected to wheel brakes are linearly pulled by a screw mechanism including a screw-threaded shaft and a nut member, there is less possibility that the cables bend and produce friction when compared to a type of electrically operated parking brake apparatus in which cables are wound around a rotational barrel (for example, JP-A-7-257356). Therefore, the electrically operated parking brake apparatus disclosed in JP-A-2000-309255 is superior in operation efficiency in pulling the cables and durability of the cables than the apparatus disclosed in JP-A-7-257356.
When the electric rotor is stopped after the electric motor is driven so as to pull the cables via the screw mechanism, the screw-threaded shaft is caused to rotate in a reverse direction by the tension of the cables, whereby the nut member is caused to move. This leads to the possibility that the brake force applied to the wheel brakes may be released. To cope with this, in the parking brake apparatus disclosed in JP-A-2000-309255, a worm reduction gear is disposed between the electric motor and the screw mechanism to prevent the reverse rotation of the screw-threaded shaft by a reverse rotation preventing function provided in the worm reduction gear.
However, while the worm reduction gear has the function to prevent the reverse rotation of the screw-threaded shaft, since the worm reduction gear is not constructed to transmit the load (i.e., the tension of the cables) from the screw mechanism to the electric motor side in a reverse fashion, it is not possible to estimate the magnitude of a brake operating force from the magnitude of the load of the motor which is indicated, for example, by current flowing to the electric motor. Therefore, there is a drawback in that a brake force cannot be generated on the wheel brakes which corresponds in magnitude to the inclination of a road surface where the vehicle is parked.
The present invention was made in view of the aforementioned problems in the related art, and an object of the invention is to provide an electrically operated parking brake apparatus which can estimate the magnitude of a brake operating force from the load of an electric motor which is generated in accordance with the load that is transmitted in the reverse fashion, while preventing the brake force applied on the wheel brakes from being loosened with the load transmitted from the screw mechanism side to the electric motor side in the reverse fashion.
With a view to attaining this object, an electrically operated parking brake apparatus, includes:
an electric motor;
a screw mechanism including a screw-threaded shaft adapted to be rotationally driven by the electric motor, and a nut member meshing with the screw-threaded shaft, the nut member being movable along the screw-threaded shaft by the rotation of the screw-threaded shaft;
a transmission member connected to said nut member for transmitting a brake operating force to a wheel brake; and
a rotation preventing unit for confining said screw-threaded shaft to a stationary portion of a vehicle body thereby preventing rotation of said screw-threaded shaft so as to hold said brake operating force by fixing a position of said nut member relative to said screw-threaded shaft.
According to one embodiment, the rotation preventing unit is constituted by an electromagnetic brake for confining the screw-threaded shaft to the stationary portion of the vehicle body. Therefore, even if a power transmission path between the electric motor and the screw mechanism is constituted by a unit for transmitting the load on the transmission members from the screw mechanism side to the electric motor side in the reverse fashion, the magnitude of brake operating force can be estimated based on the motor load of the electric motor, such as current generated on the electric motor in accordance with the load of the transmission members that is transmitted in the reverse fashion when the confinement of the screw-threaded shaft by the electromagnetic brake is released. This configuration also prevents the loss of the brake force applied to the wheel brakes by preventing the reverse rotation of the screw-threaded shaft with the electromagnetic brake while the parking brake apparatus is in operation. Thus, for example, a brake force can be generated on the wheel brakes which corresponds in magnitude to the inclination of a road surface where the vehicle is parked
In addition, since the electromagnetic brake in this embodiment confines the screw-threaded shaft to the stationary portion of the vehicle body by virtue of friction force, the inertia force of the electric motor can be controlled more easily when compared, for example, with the confinement of the screw-threaded shaft with a ratchet mechanism, whereby the nut member on the screw-threaded shaft can be controlled accurately to a target position. Furthermore, in case the magnitude of friction force is altered by controlling current that is fed to the electromagnetic brake, the confinement and release of the confinement of the screw-threaded shaft can be implemented in a moderate fashion, whereby the operating noise of the parking brake apparatus can be reduced.
Note that Bowden cables 13, 13 in an embodiment correspond to the transmission members of the invention, a housing 41 in the embodiment to the rotation preventing unit in the invention, and a ball screw mechanism 49 in the embodiment to the screw mechanism in the invention.